Field of the Disclosure
The present disclosure relates to a liquid crystal display device and a method of fabricating the same.
Discussion of the Related Art
Liquid crystal display (LCD) devices are classified into a twisted nematic (TN) mode device, an in-plane switching (IPS) mode device, and a ferroelectric liquid crystal (FLC) mode device according to a driving method.
Among these, an IPS mode LCD device includes two substrates and a liquid crystal layer between the two substrates, and changes an arrangement of liquid crystal molecules by applying an electric field parallel with the substrates, thereby displaying an image. The IPS mode LCD device has been widely used for various devices such as cellular phones or televisions because of its wide viewing angles.
In general, an arrangement of liquid crystal molecules of an LCD device is controlled such that the liquid crystal molecules are arranged along a predetermined direction when an electric field is not applied.
To control the arrangement of the liquid crystal molecules of an IPS mode LCD device, a rubbing method or a photo alignment method has been applied. In the rubbing method, an alignment layer of polyimide is formed on a substrate, and its surface is rubbed by a rayon or cotton fabric along a predetermined direction. In the photo alignment method, polarized UV light is irradiated to cause anisotropy on a surface of a polyimide film. The liquid crystal molecules are strongly restrained to a surface of the substrate by the above-mentioned process and are arranged in the predetermined direction. The technology relating to an alignment layer has been disclosed in Japanese Patent No. 2940354 (referred to herein as Patent document 1).
In Patent document 1, liquid crystal molecules are strongly restrained by an alignment layer. Therefore, when an electric field is applied to a liquid crystal layer, an arrangement of the liquid crystal molecules is not changed at once. The arrangement of the liquid crystal molecules is changed when a voltage higher than a threshold voltage is applied.
In addition, because the liquid crystal molecules are strongly restrained by the alignment layer, when the electric field is applied to the liquid crystal layer, the arrangement of the liquid crystal molecules close to the alignment layer is not changed from an initial orientation thereof, and the arrangement of the liquid crystal molecules of the liquid crystal layer in the middle (bulk) between the two substrates is changed. In an IPS mode LCD device, brightness and darkness are switched by changing retardation accompanied by a change in the arrangement of the liquid crystal molecules.
Generally, the transmittance of the IPS mode LCD device is expressed by the following equation 1.
                    T        =                              1            2                    ⁢                                    sin              2                        ⁡                          (                              2                ⁢                φ                            )                                ⁢                                    sin              2                        ⁡                          (                              π                ⁢                                                                  ⁢                Δ                ⁢                                                                  ⁢                                  nd                  /                  λ                                            )                                                          (        1        )            
Here, φ is an angle of an arrangement direction of the liquid crystal molecules with respect to an initial orientation direction when a voltage is applied, Δn is a refractive index anisotropy of liquid crystal, d is a cell gap, and λ is a wavelength of light.
From the equation 1, it is understood that the transmittance of 50% can be ideally achieved by properly selecting values of the angle φ, the refractive index anisotropy Δn, and the cell gap d (black and white type). However, design parameters of the IPS mode LCD device are determined realistically by considering all the conditions such as a driving voltage, a response time and production yield as well as the transmittance. As a result, the real transmittance of the IPS mode LCD is lowered up to half of the ideal transmittance.
Meanwhile, with the practical use of organic electroluminescent (EL) displays, the properties of LCD devices need to improve. Particularly, a decrease in power consumption and an improvement in brightness (transmittance) are some of important development issues in order to differentiate the LCD devices from the organic EL displays. According to these backgrounds, there have been demands for low voltage driving and high transmittance of the IPS mode LCD device.